The invention relates to an electric machine.
It is known that internal combustion engines in motor vehicles must be started by means of a starting device since they are not able to start by themselves. Normally, electrically operated starter motors are used for this purpose, which are connected to a voltage source via a starter relay embodied as a so-called solenoid switch. At the same time, in order to start, a pinion of the starter motor is brought into engagement with a ring gear that is usually connected to a flywheel of the internal combustion engine. Immediately after the internal combustion engine achieves sustained operation, the starter motor must be demeshed in order to prevent increased wear and powerful noise generation. If the starter must remain constantly engaged for particular reasons, then it must be designed for continuous running duty even at higher speeds.
It is likewise known that motor vehicles are equipped with a generator which on the one hand, supplies power to an electrical system of the motor vehicle and on the other hand, charges the voltage source that is required to additionally buffer the electrical system and to operate the starter motor. This generator, also referred to as a dynamo, is usually driven by a traction mechanism directly via the crankshaft of the internal combustion engine. The generator typically produces its nominal power at an average speed, but must be dimensioned so that it can also produce a sufficient electrical power even at the idling speed of the internal combustion engine in order to prevent a draining of the voltage source during long periods of idling. At the same time, however, it must be designed for the maximal speed of the internal combustion engine and thereby must be provided with sufficient cooling. The cooling at high speeds mostly takes place by means of air ventilation, which is encouraged by means of fan blades attached to it.
If an electric machine is intended to be operated alternately as both a starter and a generator, the first problem to arise is that of a speed-adapted design. During operation as a starter motor, a high starting moment and a relatively high power are required at a low speed. In order to reliably transmit such a high power, a coupling to a drivetrain of the internal combustion engine with a pair of gears is suitably selected, which is disengaged after the internal combustion engine starts. However, the generator must be in a position to assure a sufficient power output over a wide speed range and must achieve this without being at risk of overheating at the maximal speed. If the generator should be designed for a high electrical power output of the kind that is required for modern motor vehicles with a large number of electrical consumers such as servomotors, then conventional belt drives reach the limits of their performance capacity.
The prior patent application DE 197 192 13.0 describes an electric machine which can be coupled by means of a planetary gear with a transmission input shaft of a motor vehicle and which, with two different transmission stages, can be switched between operating as a starter motor for starting the internal combustion engine and operating as a generator for supplying energy to an electrical system of the motor vehicle. However, the disadvantage here lies in the provided switching via a linear adjustment path by means of a lever that must be mechanically connected and in the exclusively-axially acting friction surfaces for locking an internal gear of the planetary gear to its housing or planetary gear carrier. Because with small radiuses, the forces can be transmitted with a sufficient degree of reliability only with a high compressive force, this device requires a high actuation force on the lever and/or a voluminous construction of the planetary gear. In the above-described embodiment, it is also disadvantageous that a self-locking of the switching positions in the planetary gear is not provided. Consequently, the required high actuation forces for maintaining a desired switching position must be permanently exerted. Smaller electrically operated servomotors are thus not suitable for exerting the actuating forces. In lieu of this, for example hydraulic actuating drive mechanisms or mechanical actuating devices with sufficient leverages are necessary, which requires extra structural expenses.
The device according to the invention offers the advantage that by means of at least two drive positions and a neutral position of a planetary gear as a drive transmission for an electric machine, this electric machine can be used both as a starter for starting the internal combustion engine and as a generator for supplying power to an electrical system of the motor vehicle. The disposition of the electric machine connected in series with the planetary gear, for example in the drivetrain of the motor vehicle between a coupling and a manually shifted transmission preferably on a transmission input shaft, in particular has the advantage that the electric machine can remain in constant engagement without expensive devices for meshing and demeshing a conventional starter, and a control of the planetary gear based on fixed parameters can be carried out simply by means of the motor control.
By means of the neutral position that is provided in addition to the two drive positions, an imminent overloading of the electric machine can be prevented in an effective and simple fashion, if need be in an automated form, and in certain operating states, a reduction of the energy consumption of-the internal combustion engine can be achieved by simply breaking the operative connection. The switching of the planetary gear can take place in an effective fashion by means of a frictional fixing of an internal gear either in relation to a planetary gear carrier or in relation to a housing of the planetary gear, wherein this switching is produced by means of a mechanical switching means which can. be triggered in different ways, for example hydraulically or electromotively. Thus, a first end position can provide a locking of the internal gear in relation to the housing and a second end position can provide a locking of the internal gear in relation to the planetary gear carrier. In the neutral position, the internal gear can rotate freely, as a result of which an operative connection between the drivetrain of the motor vehicle and the electric machine is broken.
Advantageously, the two end positions or working positions of the planetary gear are geared differently. Thus, for example, this gearing can be i=5 in the first end position and i=2 in the second end position. As a result, when starting the internal combustion engine, a high moment can be transmitted at low speeds. During regular operation of the internal combustion engine, however, the electric machine operates at high speeds and can therefore supply a high initial electrical output. In addition, this permits a switching at a certain speed of the internal combustion engine, for example at 1500 RPM, which assures a sufficient electrical supply of electrical system, even in long periods of idling.
The mechanical switching of the planetary gear takes place suitably by means of an actuator which is coupled to the gearshift of the motor vehicle and can also be alternatively controlled as a function of different operating parameters. In order to achieve a compact structure and, with low actuation forces, to simultaneously achieve an effective locking of the internal gear either in relation to the housing or in relation to the planetary gear carrier, in a preferred embodiment, at least two brake shoes are provided, which encompass the internal gear radially from the outside at its greatest circumference. In this manner, high forces can be transmitted even with low actuation forces. The mechanical switching means can be electromagnetically, hydraulically, or pneumatically actuatedxe2x80x94depending on the existing vehicle equipment.
Other advantageous embodiments of the invention ensue from the remaining features mentioned in the dependent claims.